1. Field of the Invention
The present invention relates to a synchronous signal generator using a crystal oscillator, and more specifically to a synchronous signal generator for suppressing the jitter (fluctuation of a signal with time) of a synchronous signal.
2. Description of the Related Art
A synchronous signal generator is known as a pulse generator for controlling a signal with time for a computer, etc. For example, a synchronous signal generator converts the sine wave outputted from a crystal oscillator into a pulse and outputs it. Recently, to ensure the synchronization of a signal, a high precision synchronous signal is demanded, and the synchronous signal generator is requested to reduce the jitter of an output signal.
FIG. 1 is an explanatory block diagram of a common synchronous signal generator.
The synchronous signal generator shown in FIG. 1 includes a crystal oscillator 1 and a pulse converter 2. The crystal oscillator 1 includes a crystal vibrator 3 and an oscillator circuit unit 4, and forms, for example, a Corpitts type oscillator circuit. The crystal vibrator 3 is configured by, for example, enclosing an AT-cut crystal piece, an exciting electrode formed on the crystal piece, etc., in an airtight container not shown in the attached drawings. An AT-cut crystal piece (crystal vibrator 3) indicates a vibration frequency (resonance frequency) in inverse proportion to its thickness.
Then, the output signal from the crystal oscillator 1 contains a higher harmonic component fs (f2˜fn) having a multiple of the frequency of the fundamental wave component f1 as shown in FIG. 2. In this example, the oscillation frequency f of the crystal oscillator 1 is a frequency of a fundamental wave component f1 of the crystal vibrator 3, and the output waveform is substantially a sine wave output (the output is referred to as sine wave output). However, there is a shift in the frequency value of the crystal vibrator 3 and the crystal oscillator 1, between the fundamental wave component f1 and the higher harmonic component fs.
The pulse converter 2 is formed by, for example, a complementary output driver IC, and converts the sine wave output from the crystal oscillator 1 into positive/negative rectangular pulses. Normally, the sine wave output of the crystal oscillator 1 is amplified by an amplifier 5, and then input into the pulse converter 2.
However, in the synchronous signal generator with the above mentioned configuration, there is the problem of the jitter generated in the output (pulse waveform) by the pulse converter 2.
That is, as shown in FIG. 2, although the sine wave output of the crystal oscillator 1 mainly contains the fundamental wave component f1, it also contains the higher harmonic component fs for even or odd values. Therefore, the sine wave output is not an ideal sine wave that includes only the fundamental wave component f1, generates the distortion by the higher harmonic component fs, and causes the jitter as a result. That is, the smaller the higher harmonic component fs is to the fundamental wave component f1, the closer to the ideal sine wave is the sine wave output of the crystal oscillator 1.
The pulse converter 2 generates a rectangular pulse with the jitter depending on the level of the higher harmonic component fs of the output sine wave of the crystal oscillator 1. In short, the closer to the ideal sine wave the input sine wave is, that is, the smaller the level of the higher harmonic component fs is to the fundamental wave component f1, the smaller jitter of the pulse is generated by the pulse converter 2. However, although the oscillation output of the crystal oscillator 1 mainly contains the fundamental wave component f1, it contains not a small amount of higher harmonic component fs. Therefore, there necessarily occurs the problem of the jitter in the output of the pulse converter 2.